1. Technical Field
The present invention relates to a fixing device, an image forming apparatus, and a heater control method, and more particularly, to a fixing device for fixing an image in place on a recording medium, an image forming apparatus, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine incorporating several of those features, and a power supply control method for a heater used in such a fixing device and image forming apparatus.
2. Background Art
In image forming apparatuses, such as photocopiers, facsimile machines, printers, plotters, or multifunctional machines incorporating several of those imaging functions, an image is formed by transferring ink or toner onto a recording sheet such as a sheet of paper. The transferred, unfixed toner image may be subsequently subjected to a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium with heat and pressure.
Thermal fixing is employed in electrophotographic image formation wherein heat is imparted to a recording medium from a fixing member, in the form of an endless belt or roller, heated by an electrical heating element. Various types of control systems have been proposed to maintain a desired operational temperature of the fixing member for stabilizing performance of the fixing process.
One example is on-off feedback control which controls power supply to a heater according to readings of a thermometer detecting temperature of a fixing member. Comparing the detected temperature against a desired, set-point temperature, the on-off controller turns on the heater power supply where the detected temperature falls below the set-point temperature, and turns off the heater power supply where the detected temperature exceeds the set-point temperature. Although effective for its intended purposes, on-off feedback control is susceptible to delays in response time, which can cause the operational temperature to overshoot, resulting in undesired temperature oscillations or ripples around the set-point temperature.
A sophisticated type of feedback control employs a proportional-integral-derivative or -differential (PID) calculation to adjust a period of control cycle or on-time during which the heater is supplied with electricity. A PID controller is based on a control algorithm including a combination of proportional, integral, and derivative actions, which optimizes operational parameters of the heating system according to an error signal representing a difference between a detected temperature and a set-point temperature.
A drawback of PID control is that it can cause a large inrush current to flow into the heating element of the fixing process, particularly where the heater employed is one that consumes relatively large amounts of energy, such as a halogen heater. Inrush current surge results in fluctuations in a mains voltage from which the heater derives power, which causes lighting devices, such as fluorescent lamps and light bulbs, connected to the mains voltage in common with the printer, to flicker or dim upon activation of the heating element. Such flicker and dimming of lights are pronounced where the power supply control is designed with its control cycle shortened for precision PID calculation, resulting in frequent or large inrush current generated each time the heater enters a new control cycle.
Several methods have been proposed to alleviate drawbacks of PID-control heating. Some employ a phase-fired control that modulates a duty cycle, or phase angle, defining a ratio of on-time during which the heater is supplied with an alternating current (AC) within a given control cycle. Phase controllers operate by causing a switching element to turn on at an adjustable phase angle and turn off at a zero-crossing of the applied waveform voltage, or alternatively, by causing a switching element to turn off at an adjustable phase angle and turn on at a zero-crossing of the applied waveform voltage.
Specifically, the phase controller can “soft start” the heater, in which the duty cycle gradually ramps up to a constant level of 100% (i.e., the heater is fully turned on) after initial application of power during activation of the heater. The phase controller can also “soft stop” the heater, in which the duty cycle to gradually ramps down from 100% to a predetermined constant level upon final application of power during deactivation of the heater. Such soft start and soft stop capabilities effectively prevent inrush current from occurring each time the heater enters a new control cycle.
An arrangement of such phase control has been proposed, in which the phase controller employs an estimated frequency to determine an interval between zero-crossings of an AC power supply voltage. The zero-crossing interval is used to adjust the duty cycle during recovery from an energy-saving mode in which power supply to the controller is temporarily cut off, followed by calculating an actual frequency of the applied voltage as the power supply to the heater is fully turned on. Instead of initially obtaining the calculated, actual frequency, using the estimated frequency reduces the time required to initiate phase control of the heater, leading to accelerated start-up of the fixing process after recovery from energy-saving mode.